1. Field of the Invention
This invention relates to pressure differential indicators and more particularly to such pressure differential indicators that use low airflow rate indicators to indicate and allow monitoring of pressure differentials across walls.
2. Description of Related Art
Some specific spaces or rooms such as at a hospital require a proper negative pressure in the interior space or room in order to prevent germs or virus dissipating through a fissure by airflow to an exterior space outside the room such as an exterior room, hallway or corridor. The interior air pressure is lower than that at the exterior because airflow travels from a space of higher pressure to a space of lower pressure. It is well known to provide a device that keeps the interior air pressure lower than that of the exterior in order to maintain a clean and healthy environment, such a system is described in U.S. Pat. No. 5,228,306. It is necessary for the doctors, nurses, and other concerned employees and individuals to easily monitor whether or not a proper vacuum (a negative pressure differential) is being maintained in a room from outside the room such as in the hallway or corridor. Fairly sophisticated devices, usually electronically controlled, have been developed for this purpose. However, these devices are expensive to procure and maintain and subject to failure. Furthermore, because the pressure differential across such walls, particularly hospital walls, is very small, present day devices are expensive and difficult to maintain and calibrate. Therefore, the present invention has been developed which incorporates a preferably adjustable airflow duct across the wall between the low and high pressure spaces and a low airflow indicator on the side of the wall opposite the space that is to be monitored to indicate air pressure differentials and particularly low pressure differentials across walls.
Low air velocity gauges for measuring low air flow rates are well known in the art. One conventional air velocity measuring device uses a hot wire anemometer, which requires electrical power to operate and is expensive. A relatively less expensive mechanical device in the prior art, a rotational vane type that has a number of intricate moving parts that require maintenance, is still relatively expensive and is not easy to monitor by a person walking down a hospital hall. A low air velocity gauge is disclosed in U.S. Pat. No. 4,154,101 to provide a low cost velocity gauge of simplified and trouble free nature that reliably measures air velocity or draft movement in the low air flow 20-400 feet per minute rate range.